1. Field of the Invention
The present invention relates generally to measuring and testing systems. In a more particular sense, the invention relates to the art of pipe flaw detection by vibratory means in which ultrasonic waves are beamed against a test piece. Considering the invention with even more particularity, the invention relates to a device of this nature incorporating a transducer scanning means utilizing a liquid (water) couplant.
2. Description of the Prior Art
An ultrasonic inspection transducer is used to transmit high frequency vibrational energy into a piece of test material and to receive signals reflected from the material. In order to do this, the transducer must be "coupled" to the surface of the test material by a medium which will conduct ultrasonic energy efficiently into the material. The coupling medium is generally a liquid so that it can fill and, in effect, "smooth out" microscopic surface irregularities in the transducer face and on the test material surface.
When a transducer is placed directly upon the surface of the material to be tested, a thin film of oil or glycerin is typically used as the couplant. This procedure is called "contact" testing.
Another method of transducer coupling to the test material is called "immersion" testing. In this method, the part to be tested and the transducer are connected by a quantity of water. The water may be either contained in a tank which houses both the transducer and test pieces, or may be in the form of a confined column flowing in such a manner as to continuously connect the surfaces of the transducer and the test material. In the prior art such water column devices have been referred to as "water delay lines", "bubblers", "squirters" or, in the case of very short columns, "gap scanners". (The latter device may perhaps more properly be described as a contact transducer using a water couplant since it does not enable the advantages of true immersion transducers to be realized).
It is often very desirable to utilize focussed ultrasonic beams for inspection. This is especially true when the material is curved or cylindrical in shape, such as tubing and bar stock, but it is also true for certain types of testing of materials with flat surfaces. Both spherically ("point focus") and cylindrically ("line focus") focussed transducers are used. Their proper application results in concentration of the ultrasonic energy in a desired region in the test material, and, often even more importantly, control of the incidence and refraction angles of the ultrasonic wave. This control insures that the ultrasonic wave will propagate in the material in the desired mode (longitudinal, transverse, surface, or "Lamb" waves) and at the desired angle or angles with respect to the material surfaces to provide uniform and sensitive inspection for the types of discontinuities being tested for.
Although not impossible, it is difficult to focus the ultrasonic beam from a contact transducer. This is the case because focussing is usually accomplished by using curved lenses or curved transducer elements and, in general the required radius of curvature for a given desired focal length will not match the surface contour of the part to be inspected.
Focussing of immersion transducers is relatively simple since a lens of the required curvature, often made of plastic, may be cast on to or otherwise attached to the front surface of the transducer. This results in a focussed beam in the coupling water which may then be applied to the part to be tested. Of course, suitable allowance must be made for the relative velocities of propagation of the beam in water and in the test material to determine the focal distance and angle(s) of refraction in the material.
It is broadly known to provide a device for the purposes outlined above, having a self-contained liquid coupling medium. It is also known to provide for angulation of the transducer by mounting it in a spherical member or holder capable of being rotatably adjusted and thereafter locked in a selected position of adjustment. See U.S. Pat. No. 2,956,185.
However, so far as is known the prior art does not contemplate the provision of various features regarded as being of substantial importance. It is important, for example, to provide for a more precise adjustment of the transducer holder in a device of this type than has heretofore been possible. Greater precision in establishing the adjusted position of the transducer holder results in a highly desirable, correspondingly precise angulation of the focussed beam propagated by the transducer.
It is desirable, also, to provide for adjustment of the focal distance through the couplant as measured from the beam propagating surface of the transducer and the target area of the test material, in all positions to which the transducer has been precisely angulated in respect to the test piece surface.
It is a further desirable object to control precisely leakage of the liquid couplant through a gap that necessarily exists between the surfaces of the disclosed device and the test piece.
The prior art, further, is believed to have fallen short of suggesting a device that will permit the ready interchange of face plates to accommodate the device to test pieces of different sizes and contours, while at all times maintaining a precisely established gap in which the above mentioned controlled leakage factor is present.
Finally, the prior art, though broadly suggesting the provision of a confined column of water as a couplant, has been deficient in that maximum efficiency in respect to purging of air has not been achieved.